Metal detectors operate by various principles. In some constructions, only one coil is used, and this is for example part of an oscillator. As metal is approached, the voltage at the oscillation circuit of the oscillator changes. This change in voltage is then analyzed accordingly and used to determine the metal.
Other measurement principles use a coil to emit an electromagnetic pulse which generates an eddy current in the metal part that is to be detected. This eddy current is typically received by a second coil and analyzed accordingly.
According to another method, a transmitting coil is used to emit a continuous alternating field which is received by a second coil. In relation to the first coil (transmitting coil), this second coil is arranged such that, in the absence of an effect by the metal in an object to be detected, the magnetic effect of the transmitting coil is eliminated in the receiving coil. The mechanical arrangement of the transmitting and the receiving coil must be selected to be very stable, since any small displacement or deformation will immediately give rise to an output signal in the receiving coil.
In a further method disclosed in WO 2010/084000 A1 and WO 2010/133328 A1, use two transmitting coils, which are offset from one another, and continually—that is, whether or not a metal is present—regulate a receiving signal to zero with the current components. In this system, temperatures that affect the receiving coil and the preamplifier have no effect on the measured detected value. The detected value is derived from the ratio of regulated transmitted currents, with the result that, even if the transmitted currents are almost the same, the possibility that temperature affecting the coil arrangements will affect the detected value may be ruled out. Mechanical deformations of the coil arrangements merely result in an offset in the detected value, since the receiving signal remains at zero.
In the methods last mentioned above, a coil arrangement is used in which the magnetic field or fields of the transmitting coil or coils is/are eliminated in the receiving coil or coils. Only the component originating in the metal object to be detected is measured. For this purpose, the coils are arranged to overlap such that the emitted magnetic field is eliminated in the receiving coil in the absence of an effect from metal. The coils only overlap with one another over a small part of the coil surface. This part is the most sensitive region for the purpose of detection. However, the majority of the coil surfaces does not overlap, so this gives large coil systems having only a small sensitive region, which is typically also asymmetrical in relation to the overall coil system.
In the case of so-called “treasure hunting” or “mine detector” devices, such as those known from DE 43 39 419 C2, a coil arrangement is frequently used in a double D arrangement. The transmitting coil and the receiving coil are mounted partly above one another in such a way that the coefficient of mutual inductance is minimal. The coils are operated alternately as transmitting and receiving coil. In this case, the two Ds are arranged with mirror symmetry and overlapping. It is clear from FIG. 1 why only a small part of the coils may overlap. This shows a coil arrangement from the side. The field lines 1.3 emanating from a transmitting coil 1.2 are more concentrated in the center point of the coil than in the external region surrounding the coil. The arrangement in FIG. 1 is the necessary result of the fact that the same number of field lines has to pass through the receiving coil 1.1 in the internal and the external region of the transmitting coil.
A metal detector operated in pulse induction (PI) mode is known from DE 103 01 951 A9. The interaction between the primary and secondary coils is decoupled by the partial overlap of the coplanar coil systems. The decoupling is adjusted by means of mechanically displaceable masses in the region of overlap, or by electrical compensation aids, for example in the form of additional compensation signals from the generator to the receiving circuit. These compensate for the proportion of the energy transmitted to the receiving coil that is not fully decoupled. There is no “feedback” between the signal detected by the receiving coil and the compensation effect—that is, there is no closed loop.
DE 103 18 350 B3 discloses a comparable arrangement in which a plurality of coils are nested within one another such that they are offset adjacent to one another and their alternating magnetic fields overlap. The largest coil, preferably the receiving coil, determines the periphery of the coil arrangement.
From DE 36 19 308 C1 is the converse of the principle mentioned above, that is to say that a surrounding transmitting coil has two figure-of-eight shaped receiving coils, which mutually cancel their emitted field.
From DE 10 2010 005 399 A1 is known the use of mutually rotatable meanders and coils in a position sensor, or to be more precise a crank angle sensor that measures the angle of the crank shaft on a vehicle motor. The meandering, as explained in paragraph (0029), is such that the coil forms a “meandering zigzag coil pattern” that meanders in a rectangular shape. This creates rectangular portions each protruding towards the outer periphery of the regulating circuit board and having connecting portions which connect respectively adjacent rectangular portions. If the coils are rotated in relation to one another at the speed of the crank shaft of the vehicle motor, a periodic oscillation is produced which can be detected in the various constructions to enable precise control of the motor. This produces a relatively high induced electrical output, but this is used only to simplify the rotary encoder.
DE 10 2004 047 189 A1 relates to a bar sensor or stud finder which is intended for example to locate metal objects hidden in building materials. These may be reinforcing bars or conductors and pipes hidden behind a wall. In order to reduce the offset problems of the prior art that occur as a result of the coil tolerances that are otherwise usual, conductor loop systems are constructed in the form of conductor structures on a circuit board. The transmitting coils and compensation and receiving conductor loops are arranged at a height offset to one another on the circuit boards.
DE 10 2004 047 188 A1, which also refers to a stud finder, aims to change the effective number of windings of a receiving winding system by using switching means to increase the sensitivity by alternative means. The solution is extremely similar in its construction to DE 10 2004 047 189 A1.
DE 10 2009 042 940 A1 relates to a position measuring device which, in accordance with FIGS. 3, 4, operates with meandering conductor tracks. The conductor tracks of a first group of serpentine conductor tracks and the conductor tracks of a second group of serpentine conductor tracks are connected to one another electrically conductively by way of a through-hole. As a result, crossing positions of the mutually crossing transmitter conductor tracks are produced and so there is no overlapping of a transmitter coil with a receiving coil.
WO 02/091021 A1 attempts, in the case of a stud finder, to achieve as small as possible an offset signal by inductive coupling of the transmitting coil and the receiving coil and, in a concentric arrangement, by dimensioning them in respect of the number of their windings and their sizes, or dimensioning the transmitting currents in respect of phasing and amplitude, such that mutual compensation takes place. In other words, the intention is to ensure that as far as possible no signal is produced unless an object comes close, with the result that an unambiguous signal is given when an object is located.
DE 197 38 841 A1 discloses periodic meandering triangular structures in an inductive angle sensor in order to bring about a detectable change when the angle changes.
U.S. Pat. No. 5,804,963 A1 discloses the basic principle of using meandering structures in order, in the case of a position sensor, to introduce periodically changing electromagnetic changes which help to make changes in a position sensor measurable. However, the meanders are only provided in certain parts and are not intended to increase the range by way of the said refinement of the measuring variables.
The fact that the term “field lines” should not be taken literally is known to all those skilled in the art, since to a certain extent they merely illustrate more easily regions of the same magnetic strength and polarity. For this reason, in the description of the invention that follows, this term will be used to indicate magnetic density. Coils are illustrated with only one winding, for the sake of better illustration. It will be readily appreciated that the invention takes as its starting point coils having a plurality of windings, or coils “printed” on a board. An alternating signal is operational in the coils; the field lines are indicated during a single clock phase.